It has become popular to transmit data through infrared rays among a personal computer 1 of FIG. 7(a), another unillustrated personal computer, a printer 2 of FIG. 7(b) serving as a peripheral device, a notebook personal computer 3 of FIG. 7(c), a portable terminal 4 of FIG. 7(d) referred to as a PDA (Personal Digital Assistant), etc. Thus, each of the above devices includes a communication element 5 composed of a photodetector (photo-receiving/light-emitting) element, a circuit driving the photodetector element, etc. The communication element 5 is provided to each device in such a manner that its photodetecting surface positions at the device's side surface. This arrangement realizes concurrent, multiple transmission while omitting time-consuming cable connecting job.
Today, the IrDA (Infrared Data Association) adopts three standard communication methods specified below as a standard method for the infrared data communication.
1 ASK (Amplitude Shift Keying) Method
In this communication method, the modulation technique adopted by a remote controller of some kinds of electronic devices is sped up. To be more specific, as shown in FIG. 8(a), a sub-carrier wave is modulated by a certain amplitude when the transmission data exhibit "0" and by a modulation factor of 0 (zero) when the transmission data exhibit "1" per cycle period T. The sub-carrier wave has a frequency of 500 kHz and a data transmission rate of 2.4-57.6 kbps. The infrared signals can be transmitted over distances of up to 300 cm.
Using the sub-carrier wave makes this communication method advantageous because satisfactory noise immunity can be attained by extracting the band of the sub-carrier wave using a bandpass filter at the receiver's end.
2 IrDA1.0 Method
This communication method is based on a so-called SIR (Serial Infrared) method. As shown in FIG. 8(b), when the data exhibit "0" in a cycle period T, a pulse is outputted for a period of 3T/16 from the starting edge of the cycle period T. On the other hand, when the data exhibit "1", no pulse is outputted as is in the ASK method above. This communication method has a higher data transmission rate than the ASK method, for example, 2.4-115.2 kbps. The infrared signals can be transmitted over distances of up to 100 cm. Compared with the ASK method, the pulse is outputted for a shorter period per cycle period T. Thus, this communication method is advantageous in terms of saving power consumption.
3 IrDA1.1 (so-called FIR: Fast Infrared) Method
This communication method is a pulse position modulating method in which each cycle period T is divided into four segments and a pulse of T/4 wide is outputted in one segment out of these four segments to represent the data. More precisely, this communication method includes two techniques: one is a technique shown in FIG. 8(c) which has a relatively low data transmission rate of up to 1.152 Mbps; the other is a technique shown in FIG. 8(d) which has a relatively high data transmission rate of up to 4 Mbps. In both the techniques, infrared signals can be transmitted over distances of up to 100 cm.
In the technique of FIG. 8(c), a pulse is outputted in the first of the four segments when the data exhibit "0", and no pulse is outputted when the data exhibit "1".
In the other technique of FIG. 8(d), in response to 2-bit data in a cycle period T, "00", "01", "10", and "11", a pulse is outputted at the first through fourth of the four segments in each cycle period, respectively. As can be understood from its data transmission rate, this technique is advantageous in that it can transmit a great volume of data, and hence, it can transmit color image data as well.
The frequency spectrum of each transmission method is illustrated in FIG. 9. More specifically, in the ASK method, the frequency band width denoted as .alpha.1 covers a range of some hundreds kHz having its center sub-carrier frequency at 500 kHz. In the IrDA1.0 method, the frequency band width denoted as .alpha.2 covers a range of frequency from a low band to some hundreds kHz. In contrast, in the IrDA1.1 method, the frequency band width denoted as .alpha.3 covers a wide range of frequency between 60 kHz and 20 MHz.
Thus, as shown in FIG. 10, a typical conventional communication element 11 comprises a transmitting section 12 applicable for all the communication methods and two receiving sections 13 and 14. The transmitting section 12 comprises two light-emitting diodes 15 and 16 cascaded to each other and a driving circuit 17. The anode of the light-emitting diode 15 is connected to a high-level voltage +Vcc, while the cathode of the light-emitting diode 16 is connected to the output of the driving circuit 17. The input of the driving circuit 17 receives a driving signal of the light-emitting diodes 15 and 16 sent from an unillustrated modulation/demodulation section.
The receiving section 13 is used for both the ASK method and IrDA1.0 method, and comprises a photodiode 21, an amplifier 22, and a comparator 23. The photodiode 21 outputs a current corresponding to the photo-receiving level. The output current is converted into a voltage by the amplifier 22, and the same is also amplified and outputted to the comparator 23. The comparator 23 outputs a receiving signal to the modulation/demodulation section. The receiving signal shifts to the high level when the output from the amplifier 22 is equal to or higher than a predetermined level and shifts to the low level, otherwise.
In contrast, the receiving section 14 is a receiving device used for the IrDA1.1 method in a high frequency band. Like the receiving section 13, the receiving section 14 comprises a photodiode 24, an amplifier 25, and a comparator 26.
As has been explained, if the conventional communication element 11 is to be used for all the communication methods explained in FIGS. 8(a) through 8(d), the communication element 11 has to include at least two types of receiving devices: the receiving section 13 used for the ASK method or IrDA1.0 method in a relatively low frequency band; the receiving section 14 used for the IrDA1.1 method in a relatively high frequency band. This arrangement poses a problem that the manufacturing cost is increased and the communication element 11 can not be downsized.
Thus, using the conventional communication element 11 as the communication element 5 is disadvantageous in terms of further downsizing, particularly, the compact devices, such as the notebook personal computer 3 and portable terminal 4.